P–R
parasite An organism that requires coexistence
with another organism for its survival. The para-
site typically draws nourishment and other needs
from its host organism without contributing in
return to the host’s survival. Some parasites can
survive away from their hosts for limited periods
of time or defined portions of their life cycles.
Some parasites are host-specific whereas others
can adapt to various hosts.
Pathogenic parasites are those that cause INFEC-
TIONand disease. Common pathogenic parasites
include flukes, worms, and PROTOZOA (amebas).
They may infect the SKINor migrate to internal
organs such as the LUNGS, LIVER, orBRAIN, where
they often form cysts. Treatment for parasitic
infections and illnesses depends on the parasite
and the illness.
People who travel to tropical regions or areas
where community sanitation is substandard may
acquire parasitic infections otherwise uncommon
in their home regions. Many systemic parasitic
infections cause gastrointestinal symptoms such as
DIARRHEA. These infections are usually contagious,
spread through fecal–oral contact (contact with
surfaces and substances such as food or water that
are contaminated with particles of feces). Diligent
PERSONAL HYGIENE, especially HAND WASHING, and
appropriate FOOD SAFETYpractices are key preven-
tive measures.
COMMON PARASITIC INFECTIONS
AMEBIASIS BABESIOSIS
CRYPTOSPORIDIOSIS CYCLOSPORIASIS
GIARDIASIS MALARIA
microsporidiosis PEDICULOSIS
SCABIES TRICHOMONIASIS
See also BACTERIA; FUNGI; MICROBE; VIRUS.pathogen A MICROBEcapable of causing illness.
The most common pathogens are BACTERIA, fungi,
parasites, and viruses. The process through which
a pathogen, also called an infectious agent, causes
illness is pathogenesis. The body attempts to pro-
tect itself from pathogens through numerous
mechanisms, key among them being ANTIBODY-
MEDIATED IMMUNITY and CELL-MEDIATED IMMUNITY.
Vaccines and treatments with ANTIBIOTIC MEDICA-
TIONS, ANTIVIRAL MEDICATIONS, and ANTIFUNGAL MED-
ICATIONS are among the methods available to
contain and eradicate pathogens once they estab-
lish infection in the body.
See also FUNGUS; PARASITE; PROTOZOA; VACCINE;
VIRUS.pertussis An illness resulting from INFECTIONwith
the VIRUSBordetella pertussis. Pertussis is among the
childhood diseases for which routine IMMUNIZATION
is the standard of care in the United States. The
hallmark of the illness is a rapid, violent cough
that causes the person to make a “whooping”
sound when trying to breathe through the cough-
ing, hence the common term whooping cough.
The cough can be severe enough to prevent
BREATHING. Pertussis was once a leading cause of
death among children under age 5. Though
immunization has dramatically reduced infection,
pertussis may still be fatal in very young children
and very old adults. IMMUNITY, either natural (fol-
lowing infection and illness) or via VACCINE, lasts
about 12 years.
The unmistakable cough is the primary symp-
tom and begins about seven days after exposure. In
untreated pertussis, the cough worsens rapidly and
may continue for as long as eight weeks. Many peo-
ple also experience VOMITINGwith the coughing.
The doctor often makes the diagnosis on the basis348